Addresses Copilot review on PR 22982. The previous spec covered
historical reservation in the two mode contexts but lost the case
where another project's FriendlyId slug history blocks a semantic
identifier when the global mode is classic — the exact scenario
that drives the converter service's :semantic_conversion path.
- Validator moves to app/validators/projects/identifier_validator.rb
as Projects::IdentifierValidator. The path-style symbol on the
validates declaration ('projects/identifier') resolves to it.
Distinguishes domain-specific validators from globally reusable
ones (UrlValidator, JsonValidator, etc).
- Spec moves to spec/validators/projects/identifier_validator_spec.rb.
- Trim comments that explained framework conventions (validator
lookup, declaration order). Keep comments only where the WHY is
non-obvious to a Rails-fluent reader.
Kabiru Mwenja